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Eyewitness videos reveal secrets of Japan’s tsunami

Footage of Japan's devastating inundation is providing vital information that will help engineers prepare for the next big one

By Peter Aldhous

(Image: D. Phillips UNAVCO/Fritz et al. 2012 GRL)

THE world looked on in horror a year ago, as news agencies broadcast eyewitness videos of Japan’s devastating tsunami. Now researchers have used two of these videos to provide new insight into the raging currents involved. This sort of information is crucial to authorities in tsunami-prone zones worldwide as they reconsider their plans in the wake of a disaster that saw almost 20,000 people swept to their deaths.

The engineers behind the analysis are now calling for video cameras to be routinely deployed for tsunami monitoring. “We have the technology&colon; security cameras,” says Costas Synolakis at the University of Southern California in Los Angeles. “All we have to do is point them in the right direction.”

Computer simulations have already been developed that describe how the waves behave when out at sea and as they near the coast. Once a tsunami hits land, however, it is very hard to predict where the water will go.

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Investigators can record high-water marks left on buildings and other structures after the event. “But you have no idea how fast the water was moving,” says Hermann Fritz of the Georgia Institute of Technology in Savannah. That’s a big problem when it comes to designing buildings to survive the next onslaught, and planning the best evacuation routes.

After the Indian Ocean tsunami in 2004, Fritz and Synolakis analysed videos taken by survivors in Banda Aceh, Indonesia, estimating the currents at between 2 and 5 metres per second. But these videos were shot more than 3 kilometres inland, after the waves had begun to slow down.

By contrast, the Japanese fishing port of Kesennuma was on the front line on 11 March 2011, when a tsunami triggered by the magnitude 9.0 megaquake at Tohoku breached Japan’s coastal defences. Eyewitnesses who had fled to the rooftops of two tall buildings – belonging to the Japanese coast guard and the Miyagi Prefecture government (both pictured) – documented the devastation with handheld camcorders.

Fritz, Synolakis and their colleagues used the shaky videos to painstakingly reconstruct the event, recording water heights and flow rates as Kesennuma was engulfed. The researchers used laser rangefinding to match the cameras’ fields of view to their real-world coordinates, and were able to correct for the motion of the cameras to calculate rates of flow.

Precious minutes

Automated camera systems watching for tsunamis could warn areas further down the coast that something truly monstrous is on the way.

Today’s warning systems can predict when a tsunami will strike land, but aren’t good at estimating how big the waves will be when they arrive. Japan has an additional system of buoys that record vertical deflections as tsunami waves enter waters near the coast. Nevertheless, the true magnitude of the 11 March tsunami only became apparent as the waves came ashore.

It would not help the first places to be hit, but video data could trigger specific warnings of a major wave to cities further down the coast, giving vital minutes for people to escape.